An NMR imaging apparatus has a magnet section including a static magnetic field coil for producing a uniform static magnetic field H.sub.O and a gradient magnetic field coil for producing a magnetic field which extends in the same direction as the static magnetic field H.sub.0 and which has a linear gradient in each of the x, y and z directions, a transmission and reception section which is arranged to apply radio-frequency pulses (radio-frequency electromagnetic wave) to an object of inspection placed within the magnetic field formed by the magnet section and to detect an NMR signal from the object, a control and image processing section which is arranged to control the operation of the transmission and reception section and that of the magnet section and to process detected data to thereby display an image, and other sections or members.
The NMR imaging apparatus having the above-described arrangement is driven in the pulse sequence based on the two-dimensional Fourier transformation method as shown in FIG. 6 in order to perform a predetermined data collecting operation. The operation carried out at each time during the data collection is as follows.
The time T.sub.l . . . The z-gradient magnetic field g.sub.s is applied, and a 90.degree. pulse (an RF signal) is applied (see FIG. 6(b) and 6(a)). Thus, spins within a specific slice plane of the object alone are selectively excited.
The time T.sub.2 . . . In order to generate a spin echo signal during the subsequent time T.sub.4, the x-gradient magnetic field g.sub.dp is applied (see FIG. 6(d)) to give the spins a phase difference corresponding to the x-coordinate (prephase). Further, in order to obtain positional data (positional data in the y-direction) in a direction perpendicular to the gradient g.sub.pr which is applied when a signal readout operation is carried out, the y-gradient magnetic field g.sub.w(k) is applied (see FIG. 6(c)) during the time T.sub.w (&lt;T.sub.2) to give the spins a phase corresponding to the y-coordinate (warp). Further, the z-gradient magnetic field g.sub.rp is applied during the time T.sub.w (see FIG. 6(b)) in order to remove the z-direction phase shift of the spins caused in the slicing operation (rephase).
The time T.sub.3 . . . In order to generate a spin echo, a 180.degree. pulse signal is applied (see FIG. 6(a)) to invert the whole spins (inversion).
The time T.sub.4 . . . In order to obtain positional data in the x-direction, the x-gradient magnetic field (projection magnetic field ) g.sub.pr is applied (see FIG. 6(d)), and a spin echo signal is detected (see FIG. 6(e)).
The spin echo signal detected during the time T.sub.4 corresponds to one of the lines obtained by subjecting to the two-dimensional Fourier transformation the distribution of intensities of signals from the spins in the object (determined by the spin density and the relaxation phenomenon). The selection of lines is effected by means of the product of the amount of application of the y-gradient, i.e., the magnitude of the y-gradient magnetic field g.sub.w(k), and the application time T.sub.w. Accordingly, all the view data which is necessary for reconstruction of an image, i.e., a series of data shown in FIG. 7, is collected by repeating the sequence shown in FIG. 6 while varying the y-gradient magnetic field g.sub.w(k). A series of data in the read-out direction shown in FIG. 7 is observed for each view as a spin echo signal, while one of the data in the warp direction is obtained for each view.
The scanning time in the Fourier transformation method is known to be substantially proportional to the number of samples in the warp direction. On the other hand, variations in the number of samples in the read-out direction are absorbed into, for example, the stand-by time between successive views and therefore substantially independent of the scanning time. Accordingly, it is only, necessary, in order to shorten the scanning time, to reduce the number of samples in the warp direction.
The conventional NMR imaging apparatus suffers, however, from the following problems. Since an appropriate number of samples is selected from several different numbers of samples determined in advance, collection of data is not necessarily effected with an optimal scanning time, that is, a necessary, adequate and minimum scanning time, for each individual object. To the contrary, collection of data is carried out in a direction in which the scanning time increases; therefore, there is a fear of the burden on the object increasing and also a risk of an artifact being generated due to the movement of the object's body.